Numerous techniques and types of apparatus have been developed for the joining of optical fibers. It has long been known that fiber ends may be butt welded, by heating, or epoxying together. With the increased interest in optical fibers, more precise means of alignment and coupling were developed.
In an early technique disclosed by Braun et al, in U.S. Pat. No. 3,768,146 issued Oct. 30, 1973, the fibers are aligned by means of a V-groove disposed in a base plate. To provide a permanent connection, a metallic pliable sleeve is slipped over the fiber ends and crimped. This technique, while capable of providing a viable splice, requires a separate precision fixture for alignment and crimping. Moreover, the crimping operation can damage the fragile optical fibers. A high degree of care is therefore required.
The use of V-grooves in a coupler element is also known in the art. U.S. Pat. No. 4,033,668 to Presby, issued July 5, 1977, discloses the use of a two piece coupler element wherein each piece contains a V-groove for aligning the optical fibers. To form the splice, the peripheral surfaces of the fibers adjacent to the fiber ends are covered with a thin metallic coating. While the fibers are sandwiched between the V-groove in each coupler element, solder is deposited through apertures in said elements to secure the fiber ends together. This procedure, in addition to requiring alignment between the two pieces of the coupler element, requires extreme care in the metallic coating of the fiber periphery and deposition of solder to avoid the introduction of these materials on the fiber end cross section. Such a foreign material would obviously produce high coupling losses between the fibers. To avoid this possibility other techniques melt a thermoplastic material, with an index of refraction closely approximating that of the fiber to be spliced, to secure two optical fiber ends which are abutted.
Fibers have also been aligned by inserting them at an angle into a tube with a uniform noncircular cross section so that the fibers are biased into a corner of the interior cross section. The corner provides an aligning groove for the fiber ends. The technique is set forth in U.S. Pat. No. 4,057,448 which issued to C. M. Miller on Nov. 8, 1977. As with other techniques, Miller relies on epoxy to permanently retain the fiber ends.
In summary, all of the above techniques require a high degree of care to accomplish a satisfactory splice. Moreover, prior art materials, such as epoxies and plastics, have been found to age and eventually deteriorate. Therefore, these materials are not suited for use in communication systems where long term stability is required.